Many modern day electronic devices include optical imaging devices (e.g., digital cameras) that use image sensors. An image sensor may be disposed on an integrated circuit (IC) that may include an array of photodetectors and supporting logic. The photodetectors, which can correspond to individual pixels, measure incident radiation (e.g., light) corresponding to an optical image, and the supporting logic facilitates readout of digital data from the IC. The digital data output from the IC corresponds to a digitally encoded representation of the optical image.
Standard IC manufacturing processes can produce ICs with layers having dimensions, such as thickness, based, at least in part, on the function of the IC. For example, an IC may include an electrical interconnect structure, such as a stack of back end of line (BEOL) metal layers. In some embodiments, such as those using frontside illumination (FSI) techniques, the metal layers have openings (apertures) over the individual photodetectors. In backside illumination (BSI) techniques, rather than having light pass through openings in the BEOL metal layers, the sensor is illuminated from the backside (i.e., the face opposite the stack of BEOL metal layers). The optical constraints for BSI are similar to FSI, except in BSI the photodetectors are often positioned closer to microlenses that are disposed on the backside of a substrate. In BSI, the substrate is thinned down to optimize the amount of light that reaches the photodetectors; whereas in FSI the thickness of the BEOL metal layers may be reduced. Accordingly, the thickness of specific layers, such as the metal layers of an interconnect structure and/or the substrate, may be reduced based on the desired functionality of the IC.